Document behavior of setvcpus during guest boot

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1088748

The hotplugged vcpus could fail to show up in the guest if
they were added during boot, see:
https://bugzilla.redhat.com/show_bug.cgi?id=807508

Also change arbitrary to arbitrarily.

src/libvirt.c

@@ -9368,9 +9368,12 @@ virDomainSendProcessSignal(virDomainPtr domain,
  *
  * Dynamically change the number of virtual CPUs used by the domain.
  * Note that this call may fail if the underlying virtualization hypervisor
- * does not support it or if growing the number is arbitrary limited.
+ * does not support it or if growing the number is arbitrarily limited.
  * This function may require privileged access to the hypervisor.
  *
+ * Note that if this call is executed before the guest has finished booting,
+ * the guest may fail to process the change.
+ *
  * This command only changes the runtime configuration of the domain,
  * so can only be called on an active domain.  It is hypervisor-dependent
  * whether it also affects persistent configuration; for more control,
@@ -9417,7 +9420,7 @@ virDomainSetVcpus(virDomainPtr domain, unsigned int nvcpus)
  *
  * Dynamically change the number of virtual CPUs used by the domain.
  * Note that this call may fail if the underlying virtualization hypervisor
- * does not support it or if growing the number is arbitrary limited.
+ * does not support it or if growing the number is arbitrarily limited.
  * This function may require privileged access to the hypervisor.
  *
  * @flags may include VIR_DOMAIN_AFFECT_LIVE to affect a running
@@ -9429,6 +9432,9 @@ virDomainSetVcpus(virDomainPtr domain, unsigned int nvcpus)
  * is hypervisor-dependent on whether just live or both live and persistent
  * state is changed.
  *
+ * Note that if this call is executed before the guest has finished booting,
+ * the guest may fail to process the change.
+ *
  * If @flags includes VIR_DOMAIN_VCPU_MAXIMUM, then
  * VIR_DOMAIN_AFFECT_LIVE must be clear, and only the maximum virtual
  * CPU limit is altered; generally, this value must be less than or

tools/virsh.pod

@@ -1743,7 +1743,9 @@ domain is next started.
 
 If I<--live> is specified, the guest domain must be active, and the change
 takes place immediately.  Both the I<--config> and I<--live> flags may be
-specified together if supported by the hypervisor.
+specified together if supported by the hypervisor.  If this command is run
+before the guest has finished booting, the guest may fail to process
+the change.
 
 If I<--current> is specified, affect the current guest state.